The present invention relates to copper base type shape memory alloys, and more particularly to improvements in copper-aluminium type shape memory alloys.
The shape memory effect is occasionally called a heat recoverable effect which refers to phenomena that an initially thermostable shape deforms into a further thermo-unstable shape which, upon heating, returns to the initial thermostable shape. Certain types of alloys of Ni-Ti, Au-Cd, Cu-Al-Ni or the like systems have been known to possess such an effect, and applied to a particular field of technology, while development of novel heat recoverable alloys and application of them to another field are now in progress.
The shape memory effect of copper type alloys emerges as phenomena that they are heated into a single phase of beta brass type sturcture (the beta phase), and cooled down to or below a temperature at which the martensite transformation start (the M.sub.s point), preferably to a temperature below the temperature at which the martensite transformation is completed (the M.sub.f point), thereat deformed, so that, upon heating to the temperature at which the reverse martensite transformation is completed (the A.sub.f point), they resume their original shape. To such phenomena, the occurrence of the martensite transformation is essential.
However, either binary copper-aluminium alloys or binary copper-zinc alloys are impractical, since the former alloys have a very high transformation temperatures, whereas the latter too low transformation temperatures. This has led to studies about elements for lowering the M.sub.s points of copper-aluminium alloys or raising those of copper-zinc alloys. As a result, the alloys of Cu-Al-Zn, Cu-Zn-Sn, Cu-Zn-Si, Cu-Al-Mn, Cu-Al-Fe, Cu-Al-Ni, Cu-Al-Sn, Cu-Zn-Ga, Cu-Au-Zn or the like systems have already been proposed as the alloys having the shape memory effect. Among these alloys, however, only the alloys of Cu-Al-Zn system are put to practical use (see U.S. Pat. No. 3,783,037 and a Japanese Patent Application laid open for public inspection under No. 52-116720) in view of easiness with which the alloying elements are prepared and their workability.
Nonetheless, there is left much to be desired for the Cu-Al-Zn alloys because, to obtain the required properties, they should contain a considerably large amount of zinc.
To add to this, the Cu-Al-Zn alloys have a disadvantage that their shape memory properties vary in the course of production or during use. Improvements in this respect are also desired in the art. The reasons for the variation in such properties are presumed to be ascribable to dezincification occurring in the course of production or during use.